The present invention relates to a frequency variable antenna device capable of varying an operating frequency thereof.
A monopole antenna device usually operates at a frequency depending on the overall length of a pole, has an expandable structure, and is widely used in a small-sized wireless apparatus. FIG. 17 shows a conventional monopole antenna device. A monopole antenna element 1 is connected to an signal power terminal 3 through a coupling capacitor 2. When this antenna device is used as a receiving antenna, a radio wave received through the signal power terminal 3 is supplied into a radio frequency (RF) receiver such as tuner. When the antenna device is used as a transmitting antenna, a transmission signal is supplied into the antenna element from an RF transmitter through the signal power terminal 3, and is emitted into a free space as a radio wave. FIG. 18 is a characteristic diagram of a gain against a frequency of this antenna device. In FIG. 18, the axis of abscissas 4 represents the frequency, and the axis of ordinates 5 represents the gain. A level 5a on the axis of ordinates indicates the reference value of the antenna gain, and a curve 6 shows the gain characteristic of the antenna element. The gain characteristic curve 6 has a relatively uniform characteristic of gain and frequency in a wide frequency range, and however, as the frequency becomes higher, a drop 7 from the reference value 5a becomes larger. This antenna device, therefore, hardly obtain a sufficient antenna gain in a wide frequency range.
An antenna device having a high antenna gain in a desired frequency range is provided.
The antenna device includes an antenna element, a variable capacitor coupled to the antenna element, a resonance circuit including the antenna element and variable capacitor, a tuning voltage supply terminal for supplying a tuning voltage for varying a capacitance of the variable capacitor, and a signal power terminal capable of at least one of sending a signal power to the resonance circuit and receiving a signal power from the resonance circuit.